This invention relates to rigs used in oil well operations. Although it is primarily directed to service rigs used in the maintenance and overhaul of existing oil wells, it might also be adapted to use in association with rigs for drilling new oil wells.
While the operation of drilling an oil well has long been performed automatically by a drilling rig, there are operations in connection with oil well drilling or oil well servicing which require a great deal of non-productive time and man power. In the case of a drilling rig it is frequently necessary to pull the drill string out of the hole (“tripping out”) to replace the bit and to run the drill pipe back into the hole. After an oil drilling rig drills a well and installs the well casing, the rig is dismantled and removed from the site. From that point on, a mobile repair unit, or workover rig, is typically used to service the well. Servicing includes, for example, installing and removing inner tubing strings, sucker rods, and pumps. It is frequently necessary to pull out a string of production pipe to service the well or maintain downhole equipment. In either case this involves a long series of repetitive steps in which joints of pipe are withdrawn from the hole, (one or two or three at a time), disconnected by “breaking out” their threaded ends, and stored while subsequent lengths are withdrawn. The process is repeated in reverse when lengths of pipe are connected (“made up”) together and inserted one after another to replace the drill string or the production string in the hole. This is generally done with a cable hoist system that includes a traveling block that raises and lowers the aforementioned tubing strings, sucker rods, and pumps.
While running lengths of pipe, it is obvious that as more and more lengths are run into the hole, the heavier and heavier the string of pipe becomes. This puts an ever increasing strain on conventional brake drums of the cable hoist system, sometimes leading to brake failure or brake inefficiency due to heating while tripping into the hole. Installing disk assist brakes on service rigs delivers the control and safety needed in the well servicing industry. As disk assist brakes are installed on remanufactured rigs, algorithms are being developed to control the speed throughout the range of downward block travel, but these algorithms are not currently being implemented. Because the conventional drum brakes are self energizing, they are difficult to use to finely control downward speeds.
Heat is the brake's worst enemy. As hookloads get heavier and the blocks get faster, more braking action must be applied to control and stop the blocks. Bringing heavily loaded blocks to a stop from fast moving downward motion generates energy that ends up being dissipated as heat. Hot brakes have control issues, resulting in part from band stretching and when the drums get out of round. Spraying water on the brake bands is one solution used in the field. As the brakes heat, the rig has a nozzle system that is designed to cool the bands down, however this system has it problems too. Heat and water changes metallurgy and causes corrosion. This can lead to component failure and general brake failure. Therefore, it is incumbent on the drilling and service industry to avoid heating the brakes too much.
Enter the hydromatic or water brake. The hydromatic brake is usually nothing more than a water pump connected to the tubing drum. When the hookload gets high, the hydromatic brake should be engaged to both slow down and control the speed. When engaged, the falling blocks and hookload energy are dissipated into the pumping of water, thereby delaying the tubing drum brake heating. However, while the hydromatic brake system can reduce conventional brake wear, it must be used to be effective. One drawback of the hydromatic is the slowing down of the running speed. When the energy of the downward moving block is transferred to the brake, the rig experiences a loss of freefall and therefore a slowing effect. As a result of this, an operator or driller will not engage the hydromatic until it is needed or mandated by standard operating procedures. If he is in a hurry to trip into the hole, he is more likely to delay the brake engagement. Often times drilling or rig operators will not engage the brake, and thus it would be desirable if an automated system was developed to automatically engage the hydromatic brake when appropriate.